An organic electroluminescent element (it is also referred to as an organic EL element hereafter) is a whole solid element comprised of an organic layer having a thickness between electrodes of only 0.1 μm or so, and light emission van be attained by relatively low voltage such as 3 to 20 V, and is a technology expected to realize as a flat surface display or lighting device.
Further the an organic EL element utilizing phosphorescence emission recently found is considered to realize principally around four times emission efficiency in comparison to a conventional organic EL element utilizing fluorescence emission, and research and development of the emission element such as layer arrangement and electrodes in addition to the development of the materials around the world.
The constitution of the organic EL element has a simple constitution such that an organic layer is interposed by a transparent electrode and a counter electrode, and is composed of extreme less number of parts in comparison with a liquid crystal display which is representative of flat surface display, and therefore the production cost is expected to be lower. However the performance and cost are far from the liquid crystal display. It is considered that the low productivity is the cause of the cost in particular.
Almost of all commercialized organic EL is produced by so called a vapor deposition method in which a layer is formed by depositing by evaporation of a low molecule weight material. This method is very excellent in the efficiency and lifetime since the low molecular weight compounds easy to purify can be used for the organic EL material (that is, a high purity material is easy to obtain), and it is easy to produce laminate structure. However the apparatus for forming the layers is restricted since the evaporation is conducted under high vacuum condition of 10−4 Pa or lower, and this method has deficiency of low through put because that it can be only applied to a substrate having small area and takes long time for forming layers of multiple layer lamination.
It is problematic in case of applying the lighting usage or large area electronic display, and this is one of causes not to utilize in these applications.
Coating method in which an organic layer is produced by such a process as spin coating, ink jet, printing, and spray coating can form films in the normal pressure, and further is suitable to form a uniform film in large area.
The coating method has characteristics that non-uniform light emitting is not apt to generate for large format element, as multiple organic materials can be precisely mixed and it is easy to adjust, for example, the ratio of the dopant to the light emission material, since necessary materials such as polymer materials and or low molecular materials are coated to form a thin layer by making them solution or dispersion. Therefore, it is very advantageous in production cost.
Materials used in the coating method include those of high molecular weight materials and low molecular weight materials, and the high molecular weight materials are difficult to use since it is difficult to purify and very slight amount of impurity causes large reduction in emission lifetime, particular in the organic electroluminescence element.
Conventionally known low molecular weight hole transport materials applicable to a hole transport layer are disclosed in, for example Patent Document 1. Hole transport layer was formed via coating method employing these compounds and the performance of the element was evaluated. They are found to have problems that working voltage rises in comparison with elements produced by evaporation method, and further, an upper layer can not be formed on the lower layer as the hole transport layer of the lower layer was dissolved by solvent of the upper layer when the upper layer is applied by coating method by an examination of these material. To dissolve the problems, for example, a technology using triarylamine derivative having a polymerizable functional group is disclosed in, for example, Patent Document 2 and Patent Document 3.
It was found that there remains a problem that durability against solvent is insufficient and the upper layer can not be laminated since cross linking density between the compounds in the layer becomes low when the number of the polymerizable functional group is small, as a result of a study in detail by the inventor of the invention. On the other side, when the number of the polymerizable functional groups is increased, energy at the cross linking by light or heat becomes large and inclination of remaining the polymerizable functional groups without relating to cross linking, and therefore it is one of the cause to deteriorate the lifetime of the element since the remaining polymerizable functional groups have high activity.
Examples using a combination of triaryl amine having polymerizable functional group and a compound polymerizable compound having structure other than triarylamine in the hole transport layer is disclosed in, for example, Patent Document 4. However there is a problem that the transport activity having the hole transport layer is inhibited and the carrier transport in the hole transport layer is restrained when large amount of the compound polymerizable compound having structure other than triarylamine is used mixed.